Manuel Vázquez Montalbán has only half an hour to spare in the offices of his literary agent at the heart of Barcelona's uptown business district. He is hot property now, the most interviewed writer in Catalonia. A communist who had been tortured and imprisoned under the Franco dictatorship, he began to write detective novels in the 1970s. "We were so bored with what others and we ourselves were writing that we did what anyone in this situation would do: write what we'd like to read," he says, 25 years on.

His 15 crime novels featuring Pepe Carvalho have sold well over a million copies and are the most widely read record of Barcelona's transformation from seedy port to tourist metropolis. The latest to be translated into English is The Angst-ridden Executive , which actually dates from 1977, and is Vázquez Montalbán at his most radical.

Montalban was born in 1939, the year Franco's armies, followed by a vengeful Catalan bourgeoisie, marched into the city. He was brought up in the Old City, where many children had absent fathers, either imprisoned - as was his own father - in exile or dead in the war, and just a few streets up from the red-light district where thousands of impoverished women were forced to work. In The Pianist , one of his several excellent non-detective (or "open", as he says) novels, he wrote: "We carried the Civil War defeat on our backs like a dead body."

"After my father came out of prison, he refused to talk or hear anything about politics," he says, Nevertheless, Montalbán became active in the university underground at the end of the 1950s. Tortured and imprisoned by the dictatorship, he was granted an amnesty in 1963.

His fourth Carvalho novel, 1979's Southern Seas , won Spain's biggest literary prize, the Planeta, and became an international bestseller. As well as the Carvalho series, he has written opinion articles, political history, essays on popular culture, a cook-book and a number of novels. Yet he has been pigeon-holed as a genre writer. "It used to bother me, because it was a kind of racism, snobbery. Critics sniffed 'Carvalho, oh yes, very capable', whereas in fact several Carvalho novels, because of their tempo and linear development, were much more difficult to write than Galíndez , for example."

Despite his fame, Montalbán remains a dissident, faithful to his past and to the PSUC - the Catalan Communist Party - that he joined 40 years ago. He has consistently criticised the Socialist Party that has ruled Barcelona since 1979 for its failure to use the 1992 Olympics-led economic bonanza to tackle poverty. "Behind the clean-up of old Barcelona, there is a speculative operation designed to expel the Indians to a new reservation on the outskirts . . . The city's shit is being swept under the carpet. No solutions are offered to the people living in this shit, so that they can get out of it for good."

Montalbán's detective novels are often likened to Raymond Chandler's. Carvalho is, like Marlowe, a "common man". Equipped with the wisecracks and pistol required to deal with an evil urban world, he is a moral man walking the mean streets. Montalbán does for Barcelona what Chandler did for Los Angeles - he exposes the criminal power relationships beneath the facade of democracy. But he himself rejects such comparisons: "I had read very few noir novels when I started the Carvalho series. My books are not Mediterranean derivatives or copies of Chandler. The real difference lies in Marlowe's lack of culture. His greatest interest is what Joe Dimaggio did in the baseball game, whereas Carvalho has a cultural and political background, a personal history..."

Montalban's serious purpose in the Carvalho novels is to chronicle the new Spanish democracy almost as it happens. "I try to examine themes that exist in real time. In each book I am looking at different stages: in The Angst-ridden Executive the illusions of the transition from dictatorship to parliamentary democracy; in Murder in the Central Committee the crisis in the Communist Party; in Offside sport and the fleeting nature of media fame; in An Olympic Death entrepreneurs whose athleticism consisted in lining their own pockets as quickly as possible."

Crime novels just cannot carry this sort of political weight unless they work as entertaining thrillers. Not all Montalbán's do; but in his energetic best, social and political purpose is very much part of the story, and the detective's cooking, sexual or book-burning quirks become more than frothy additions.

These tough-guy books are also gentle: much of their attractive, lyrical melancholy is based on Montalban's subtle appreciation of loneliness and time passing. He deals with loneliness tenderly, particularly with Carvalho himself and his "family", an odd, plebeian group of dependants from the old town - his call-girl girl-friend Charo, the boot-black Bromide, and the car-thief Biscuter.

It is noon. My half-hour is up. The author plods off next door to more boring questions. I look round the conference room, lined by thick red box-files of proof copies of books. On their spines are picked out in gilt the names of many of the most famous Spanish and Latin American novelists of the last 40 years: García Márquez, Cortázar, Onetti, Marsé...and Montalbán himself.

He is one of the elite, yet he is still very much with us, engaging with present realities. "Barcelona's not a bad place to live,"he says. "But there's been an enormous operation in these 20 years to instil disenchantment, to depoliticise people." His sardonic, sceptical eye is an antidote to disenchantment. He is the modern committed writer, entertaining his readers as he reveals what lies beneath Barcelona's glittering carpet.